1. Technical Field of the Invention
The present invention relates to a novel process for the preparation of picric acid.
2. Description of the Prior Art
Picric acid, or 2,4,6-trinitrophenol, is a known compound having a wide variety of industrial applications and, more particularly, for the production of explosives, medicaments, colorants and other applications (Tadeusz Urbanski, Chemistry and Technology of Explosives, Pergamon Press, pp. 498 (1964)).
Two principal routes are described in the literature for the preparation of picric acid, namely, sulfonitration of phenol or nitration of a dinitrophenol which is prepared via the hydrolysis of chlorodinitrobenzene.
The first entails a two-step process, comprising a step for sulfonation of the phenol and then a step for nitration of the sulfonated phenol thus obtained.
However, such a process suffers from a number of disadvantages, i.e., the sulfonation operation is lengthy and provides a low level of productivity and the nitration sequence is then effected in a reaction medium which is already dilute, which results in a decrease in productivity.
The other technique for preparing picric acid also comprises a plurality of steps, namely, nitration of monochlorobenzene to provide chlorodinitrobenzene, followed by hydrolysis of the product thus obtained, and then nitration of the dinitrophenol by means of a mixture of nitric acid and oleum.
This latter process is also not satisfactory as it too is lengthy, complicated and promotes pollution by virtue of the formation of sodium chloride in the hydrolysis step. Dealing with the saline aqueous effluents thus formed presents a serious drawback.